Hi and welcome!
There are no standard widths for seat belts or passenger seats, so that shouldn't cause any problems.
It looks like there is plenty of space width-wise, and very little front to back. The other challenge is that the horizontal seat has very little depth, so overhang could be an issue.
For the 8 month old, I would look at the Cosco Scenera NEXT. It's very compact and very affordable.
For the 3 year old, I would go one of 2 ways:
1) Figure out if there is any way to retrofit a tether anchor. (Can someone with LATCH manual confirm?)
2) Rear face. How tall/heavy is s/he?
It's legal to use a forward facing harness without a tether in the US, but it's strongly recommended and very important for safety.
As for keeping seat belts tight, your outboard seats have seat belts with switchable retractors. That means you pull the shoulder belt out until it stops, buckle the car seat in, then feed the shoulder belt back into the retractor. It may make a clicking noise as it goes back in.
I don't see a seat belt for the center position at all in the images I found. Is there a lap only belt there that maybe falls down where it doesn't obviously exist in low resolution photos? If so, those work great for installing rear facing seats. If they have a tail on the end, just buckle and pull the tail until it's tight.